1. Technical Field
The present invention relates to an optical filter which is used in electronic optical devices such as video cameras, and has functions such as infrared ray removal, and in particular relates to a chamfered optical filter.
2. Background Art
FIG. 5 and FIG. 6 include drawings for describing a conventional example of an optical filter, wherein FIG. 5A is a perspective view thereof, and FIG. 5B is a partial cross-sectional view thereof taken along arrows A-A in FIG. 5A. FIG. 6 includes drawings of a portion of a glass wafer, wherein FIG. 6A is a perspective view thereof before splitting, and FIG. 6B is a perspective view thereof after splitting.
An optical filter 1 shown in FIG. 5A comprises an optical plate 3 having a chamfered section 2 formed on the outer peripheral edge section of one principle (main) surface thereof, and an infrared-cutting optical thin film 4 formed on the one principle surface of the optical plate 3. The chamfered section 2 is C-chamfered as shown in FIG. 5B.
In this type of optical filter, first, the infrared-cutting optical thin film 4 is formed on a glass wafer 5 by means of a sputtering method or a vapor deposition method. Next, as shown in FIG. 6A and FIG. 6B, the glass wafer 5 is split along the split line B-B shown in FIG. 6A into optical plates 3 of a predetermined size, using a dicing blade 6. Finally, the chamfered section 2 is formed on the outer peripheral edge section of the one principle surface of each of the optical plates 3 by means of polishing or sandblasting with use of loose abrasive grains or fixed abrasive grains, thereby providing the optical filter 1. As shown in FIG. 7, the chamfered section 2 may also be formed on both principle surfaces of the optical plate 3.
As described above, the optical filter 1 has the chamfered section 2. Therefore, there is a reduced risk of breakage of the outer peripheral edge section of the optical filter 1 and of broken pieces becoming attached onto the principle surface of the optical filter 1, which consequently impair functions of the optical filter 1. This is because the outer peripheral edge section of the optical filter 1 becomes sharp-edged if the chamfered section 2 is not present, and the outer peripheral edge section may be easily broken if the optical filter 1 comes in contact with an object of some kind. However, by forming the chamfered section 2 on the optical filter 1, the sharp-edged sections are eliminated, and consequently breakage of the outer peripheral edge section is reduced (Patent Document 1: Japanese Unexamined Patent Publication No. 2004-177832).
However, there is a problem in the optical filter 1 configured in the way described above in that the optical thin film 4 is partially removed when splitting the glass wafer 5 shown in FIG. 6A (refer to partially removed optical thin film sections 7a and 7b in FIG. 6B). This problem arises from the stress of the dicing blade 6 which is used for splitting the glass wafer 5, being placed on the optical thin film 4 in the region in the vicinity of the split line B-B of the glass wafer 5.
Furthermore, the chamfered section 2 of the optical filter 1 shown in FIG. 5A and FIG. 5B is formed using a mechanical processing method such as polishing and sandblasting. Therefore, there is a problem in that the stress is placed on the chamfered section 2 and consequently micro-cracks 8 are formed in the chamfered section 2 (refer to FIG. 5B). If a force of some kind is externally placed on these cracks 8, the optical filter 1 is easily broken and broken pieces become attached onto the principle surface of the optical filter 1, impairing the functions of the optical filter 1.
Here, Patent Document 1 discloses an optical filter 1 in which a thick section 9 is provided in the center section of the dicing blade 6 (refer to FIG. 8A) and the inclination 9a of the thick section 9 is made equivalent to the inclination of the chamfered section 2, to thereby form the chamfered section 2 when splitting the glass wafer 5. Accordingly, there is no need for performing a separate chamfering process, and consequently productivity can be improved.
However, the stress of the dicing blade 6 is still placed on the optical thin film 4 in the region in the vicinity of the split line of the glass wafer 5 (refer to the line B-B in FIG. 6A), and there remains the problem of the optical thin film 4 being partially removed (refer to the partially removed optical thin film sections 7a and 7b in FIG. 8B). There is also a problem of micro-cracks being formed in the chamfered section 2 because the chamfered section 2 is formed in a mechanical processing method using the dicing blade 6.
Patent Document 2 (Japanese Examined Patent Publication No. 4148139) discloses an optical filter 1 in which the cross-section of the chamfered section 2 is of an arc shape which is convex in an outward direction of the optical filter, that is, an R-chamfered optical filter 1 (refer to FIG. 9). However, still the glass wafer 5 is split using a dicing blade 6, and the chamfered section 2 is formed in a mechanical processing method. Therefore the above problems remain unsolved.
An object of the present invention is to provide an optical filter that prevents the optical thin film from being partially removed and prevents micro-cracks occurring in the outer peripheral edge section of the principle surface, and that has a high yield rate.
Moreover, Patent Document 3 (Japanese Unexamined Patent Publication No. 2008-58427) discloses an optical filter in which there is formed an arc-shaped chamfered section, which is cross-sectionally concave in an inward direction of an optical plate. Hereunder is a specific description of this type of optical filter, with reference to FIG. 10.
The optical filter shown in FIG. 10A is an optical low-pass filter 13 in which an infrared-cutting glass 12 is laminated on a crystal birefringent plate 10, using an adhesive agent 11. In the outer peripheral edge sections of the principle surfaces of the crystal birefringent plate 10 and the infrared-cutting glass 12 opposing each other, there are formed arc-shaped chamfered sections 2, which are cross-sectionally concave respectively in an inward direction.
Moreover, on the reverse surface to the principle surface of the crystal birefringent plate 10 opposing the infrared-cutting glass 12, there is formed an anti-reflection film 14. Furthermore, on the principle surface of the infrared-cutting glass 12 opposing the crystal birefringent plate 10, there is formed an infrared-cutting coating layer 15.
Moreover, the optical filter shown in FIG. 10B is an optical low-pass filter 13 in which a crystal birefringent plate 10 is laminated on both of the principle surfaces of an infrared-cutting glass 12, using an adhesive agent 11. In the outer peripheral edge section of both of the principle surfaces of the infrared-cutting glass 12 there are formed arc-shaped chamfered sections 2 which are cross-sectionally concave towards the inner side of the infrared-cutting glass 12, and on one of the principle surfaces there is formed an infrared-cutting coating layer 15. Here, the crystal birefringent plate 10 is of a configuration the same as that of the crystal birefringent plate 10 shown in FIG. 10A.
These chamfered sections 2 of the optical low-pass filter 13 are formed by means of wet-etching. Therefore, micro-cracks do not occur in the chamfered sections 2. The present invention focuses attention on the shape of the chamfered section 2 which enables this type of effect.
However, the purpose of the chamfered sections 2 of the optical filter 1 shown in FIG. 10 is to prevent the adhesive agent 11 from sticking out the side surface (refer to Patent Document 3, paragraph [0012]). Therefore, even if the shape of the chamfered section 2 shown in FIG. 10 is applied as is, the chamfering effect where breakage in the edge section can be prevented, may not always be obtained.
For example, as shown in FIG. 11, in a case of forming an optical filter in which the crossing angle between the concave surface where the chamfered section 2 is formed and the principle surface of the optical plate 3 (reference symbol θC in FIG. 11) and the crossing angle between the concave surface and the side surface of the optical plate 3 (reference symbol θD in FIG. 11) are respectively 90°, the crossing sections between the concave surface, and the principle and side surfaces respectively become sharp edges and are likely to get broken, that is to say, the effect of chamfering cannot be obtained. In those cases where the optical plate is a crystal, it has anisotropy with respect to wet-etching. Therefore a uniformly angled concave surface cannot be formed in the outer peripheral edge section.